Method for manufacturing electroseamed metal tube, and electroseamed metal tube

ABSTRACT

Provided is a method for manufacturing an electric resistance welded metal pipe by butting side ends of a metal strip against each other and then welding the side ends by high frequency heating to manufacture an electric resistance welded metal pipe, each side end being provided with an inner surface side corner portion located on an inner surface side of the electric resistance welded metal pipe, wherein the method comprises a step of forming an inclined surface at the inner surface side corner portion before butting the side ends of the metal strip; and wherein the side ends are butted and welded to each other such that the inclined surface remains on an excess metal of the metal pipe after electric resistance welding and a discharged metal is not welded to the excess metal.

TECHNICAL FIELD

The present invention relates to a method for manufacturing an electricseamed tube (electric resistance welded metal pipe) by butting side endsof a metal strip against each other and then welding the side ends bymeans of high frequency heating, and to the electric resistance weldedmetal pipe.

BACKGROUND ART

A method for manufacturing this type of electric resistance welded metalpipe conventionally used and the electric resistance welded metal pipeare disclosed, for example in patent document 1. The prior art carriesout electric resistance welding after providing a tapered shape witheach side end of the metal strip, in order to remove a penetrator fromwelded portions.

CITATION LIST Patent Literature

Japanese Patent Application Publication No. 2008-105075 A

SUMMARY OF INVENTION Technical Problem

Even if the penetrator has been removed from the welded portions as inthe prior art, cracking might take place in the welded portions of theelectric resistance welded pipe when subjecting the electric resistancewelded pipe to severe processing such as 90° bending.

As a result of detailed studies of cracking of the welded portions, thefollowing new findings have been obtained. That is, a discharged metaldischarged from the welded portion has decreased toughness due to aninfluence of heat, and has tended to generate cracking due to severeprocessing. Observation of the welded portions where the cracking hasbeen generated has indicated that the cracking generated in thedischarged metal has been propagated to a base material through anexcess metal.

The present invention has been made to solve the above problems. Anobject of the present invention is to provide a method for manufacturingan electric resistance welded pipe and the electric resistance weldedpipe, which can avoid the propagation of the cracking to the basematerial even if the cracking is generated in the discharged metal.

Solution to Problem

A method for manufacturing an electric resistance welded metal pipeaccording to the present invention is a method for manufacturing anelectric resistance welded metal pipe by butting side ends of a metalstrip against each other and then welding the side ends by highfrequency heating to manufacture an electric resistance welded metalpipe, each side end being provided with an inner surface side cornerportion located on an inner surface side of the electric resistancewelded metal pipe, wherein the method comprises a step of forming aninclined surface at the inner surface side corner portion before buttingthe side ends of the metal strip; and wherein the side ends are buttedand welded to each other such that the inclined surface remains on anexcess metal of the metal pipe after electric resistance welding and adischarged metal is not welded to the excess metal.

The electric resistance welded metal pipe according to the presentinvention comprises: a metal pipe body having a butted welded portion;an excess metal; and a discharged metal, the excess metal and thedischarged metal being formed on the butted welded portion, wherein theexcess metal comprises an inclined surface, and the discharged metal isnot welded to the excess metal.

Advantageous Effects of Invention

According to the method for manufacturing the electric resistance weldedmetal pipe and the electric resistance welded metal pipe of the presentinvention, the side ends are butted and welded to each other such thatthe inclined surface remains on the excess metal of the metal pipe afterelectric resistance welding and the discharged metal is not welded tothe excess metal, so that the propagation of the cracking to the basemetal can be avoided even if the cracking is generated in the dischargedmetal.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view showing an electric resistance weldedmetal pipe manufacturing apparatus for carrying out a method formanufacturing an electric resistance welded metal pipe according toEmbodiment 1 of the present invention.

FIG. 2 is a front view showing a main part of a crashing apparatus inFIG. 1.

FIG. 3 is an enlarged cross-sectional view of a side end of a metalstrip in FIG. 2.

FIG. 4 is an enlarged cross-sectional view of a welded portion of ametal pipe after electric resistance welding by a welding apparatus inFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments according to the present invention will bedescribed with reference to the drawings.

Embodiment 1

FIG. 1 is an explanatory view showing an electric resistance weldedmetal pipe manufacturing apparatus for carrying out a method formanufacturing an electric resistance welded metal pipe according toEmbodiment 1 of the present invention. The electric resistance weldedmetal pipe manufacturing apparatus in FIG. 1 is an apparatus formanufacturing an electric resistance welded metal pipe 2 from a metalstrip 1 in a form of flat plate. The electric resistance welded metalpipe manufacturing apparatus is provided with a crushing apparatus 3, aforming roll group 4; and a welding apparatus 5.

The crushing apparatus 3 is an apparatus for correcting shapes of theside ends of the metal strip 1. The crashing apparatus 3 will bedescribed later with reference to the drawing.

The forming roll group 4 is configured by a plurality of forming rolls,and is disposed on a downstream side of the crashing apparatus 3 along aconveying direction 1 a of the metal strip 1. The metal strip 1 in whichthe shapes of the side ends have been corrected by the crushingapparatus 3 is gradually curved by the forming roll group 4 to form anopen pipe 4 a. The open pipe 4 a is a pipe which is obtained by buttingthe side ends on both sides along a width direction of the metal strip 1against each other and which has a C-shaped cross section having anopening at a portion where the side ends are butted.

The welding apparatus 5 is disposed on a downstream side of the formingroll group 4 along the conveying direction 1 a of the metal strip 1 andhas a heating coil 50 and a squeezing roll 51. The heating coil 50 is acoil through which a high frequency current is passed. When the openpipe 4 a passes through the inside of the heating coil 50, the side endsof the metal strip 1 forming the open pipe 4 a is heated (highfrequency-heated) and melted. The squeezing roll 51 is configured by apair of side rolls that restrain an outer periphery of the open pipe 4a, and butts the side ends of the metal strip 1 in the open pipe 4 a,melted by heating in the heating coil 50, against each other to join(weld) them.

That is, in the electric resistance welded metal pipe manufacturingapparatus according to the present embodiment, the side ends of themetal strip 1 are butted against each other, and the side ends are thenwelded by high frequency heating to manufacture an electric resistancewelded metal pipe. Although not shown, the downstream of the weldingapparatus 5 may be provided with a correcting apparatus for correctingthe size and shape of the metal pipe after electric resistance welding(a metal pipe immediately after the welding apparatus 5). When thecorrecting apparatus is provided, the metal pipe after correction in thecorrection apparatus corresponds to the electric resistance welded metalpipe 2. On the other hand, when no correcting apparatus is provided, themetal pipe after electric resistance welding corresponds to the electricresistance metal pipe 2.

FIG. 2 is a front view showing a main part of the crashing apparatus 3in FIG. 1, and FIG. 3 is an enlarged cross-sectional view of the sideend 10 of the metal strip 1 in FIG. 2. As shown in FIG. 2, the crushingapparatus 3 is provided with a pair of crushing rolls 30 for correctingthe shapes of the side ends of the metal strip 1. The crushing rolls 30are disposed apart from each other along the width direction 1 b of themetal strip 1. The metal strip 1 is passed through a space between thecrushing rolls 30.

Each side edge 10 of the metal strip 1 before passing through thecrushing rolls 30 is provided with an inner surface side corner portion10 a located on the inner surface side of the electric resistance weldedmetal pipe 2. Each crushing roll 30 is provided with a crushing surface30 a that extends and inclines to a rotation axis of the crushing roll30. When the metal strip 1 is passed through the space between theclashing rolls 30, the crushing surface 30 a is pressed against theinner side corner portion 10 a to from an inclined surface 10 b on theinner side corner portion 10 a. As shown in FIG. 3, the inclined surface10 b has a width W along the width direction 1 b of the metal strip 1,and extends and inclines by an angle θ relative to an inner peripheralsurface 10 c of the metal strip 1. The formation of the inclined surfaceis preferably carried out by pressing the crushing roll against theinner surface side corner portion. In addition, it may also be carriedout by the formation with fin pass rolling of a forming roll, cuttingwith a cutting tool, grinder grinding, or a combination of thereof.

It should be noted that the metal strip 1 used for manufacturing of theelectric resistance welded metal pipe 2 may be cut out from a biggermetal strip, and burring and sagging may occur in the side ends 10during the cutting out of the metal strip 1. When the burring andsagging occur on the side ends 10, it is preferable that the burringside is the inner surface side corner portion 10 a in order to form theinclined surface 10 b stably.

FIG. 4 is an enlarged cross-sectional view of the welded portion of themetal pipe after electric resistance welding by the welding apparatus 5in FIG. 1. As shown in FIG. 4, the welded portion of the metal pipeafter the electric resistance welding includes a joined portion 101 anda heat affected portion 102. The joined portion 101 is a portion wherethe side ends 10 of the melted metal strip 1 are joined to each other.The heat affected portion 102 is a portion affected by heating duringmelting of the side ends 10. The joined portion 101 and the heataffected portion 102 have structures different from a non-heat affectedportion 100 due to the influence of heat.

From the joined portion 101, a discharged metal 101 a protrudes towardan inner side of the metal pipe. The discharged metal 101 a is asolidified molten metal resulting from pushing of the molten metal fromthe joined portion 101 when the side ends 10 of the metal strip 1 arepressed against each other. The formation of the discharged metal 101 aplays a role of discharging an oxide (a penetrator) contained in themolten metal from the joined portion 101.

On the heat affected portion 102, an excess metal 102 a is formed. Theexcess metal 102 a is a raised portion of the metal softened in the heataffected portion 102 by a pressure when the side edges 10 of the metalstrip 1 are pressed against each other.

In the method for manufacturing the electric resistance welded metalpipe according to the present embodiment, when butting the side ends 10of the metal strip 1 against each other to weld them in the weldingapparatus 5, the side ends 10 are butted and welded to each other suchthat the inclined surface 10 b remains on the excess metal 102 a of themetal pipe after electric resistance welding and the discharged metal101 a is not welded to the excess metal 102 a as shown in FIG. 4. Thatis, the electric resistance welded metal pipe 2 according to the presentembodiment includes: a metal pipe body having a butted welded portion;the excess metal 102 a; and the discharged metal 101 a, the excess metal102 a and the discharged metal 101 a being formed in the butted weldedportion, in which the excess metal 102 a has the inclined surface 10 band the discharge metal 101 a is not welded to the excess metal 102 a.

The discharged metal 101 a has decreased toughness due to the influenceof heat. Therefore, when the electric resistance metal pipe 2 issubjected to severe processing such as 90° bending for example, crackingmay occur in the discharged metal 101 a. As described above, the sideends 10 are butted and welded to each other such that the inclinedsurface 10 b remains on the excess metal 102 a of the metal pipe afterelectric resistance welding and the discharged metal 101 a is not weldedto the excess metal 102 a, so that even if the cracking occurs in thedischarged metal 101 a, the propagation of the cracking to the basematerial can be avoided. It should be noted that the discharged metal101 a may be in contact with the excess metal 102 a as long as thedischarged metal 101 a is not welded to the excess metal 102 a. Thepresence or absence of welding of the discharged metal 101 a to theexcess metal 102 a can be confirmed by observing the cross section ofthe welded portion in the metal pipe after electric resistance welding.

Whether or not the inclined surface 10 b remains on the excess metal 102a is affected by a heat coefficient and a width W of the inclinedsurface 10 b. The heat coefficient refers to a coefficient representedby the equation: (I×V)/(T×S), in which V represents a voltage [kV]applied to the heating coil 50, T represents a thickness [mm] of themetal strip 1, and S represents a pipe manufacturing rate [m/min] (aconveying rate of the metal strip 1), and the heat coefficient can beused as an index representing an amount of heat applied to the side ends10 when butting the side ends of the metal strip 1 against each other toweld them.

As the heat coefficient is higher, the discharged metal 101 a is larger.Therefore, the inclined surface 10 b is less likely to remain. Further,as the width W of the inclined surface 10 b is wider, an amount of thedischarged metal 101 a is lower. Therefore, the inclined surface 10 b ismore likely to remain.

Further, whether or not the discharged metal 101 a is not welded to theexcess metal 102 a is affected by the width W of the inclined surface 10b and the angle θ. As the width W of the inclined surface 10 b is wider,the amount of the discharged metal 101 a is lower. Therefore, thedischarged metal 101 a is likely to be not welded to the excess metal102 a. Furthermore, as the angle θ of the inclined surface 10 b islarger, the butted area is smaller. Therefore, the discharged metal 101a is less likely to be not welded to the excess metal 102 a.

Therefore, by adjusting the angle θ and the width W of the inclinedsurface 10 b, the heat coefficient as well as an upset amount so as tofall within predetermined ranges, the inclined surface 10 b can remainon the excess metal 102 a of the metal pipe after electric resistancewelding and the discharged metal 101 a can be non-welded to the excessmetal 102 a.

The upset amount corresponds to an pressing amount of the side ends 10when the side ends 10 are joined, and can be represented by a differencebetween a value obtained by subtracting a spacing distance between theend faces of the open pipe 4 a at a position just before beingintroduced into the squeezing roll 51 from the outer peripheral lengthof the open pipe 4 a at the same position, and the circumferentiallength of the metal pipe after electric resistance welding. As theposition just before being introduced into the squeezing roll 51, aposition that is 1 m before the squeeze roll 51 can be set.

As a tensile strength of the metal strip 1 is higher, the cracking inthe discharged metal 101 a is likely to be generated. In particular,when the tensile strength of the metal strip 1 is 700 MPa or more, thecracking is significantly generated in the discharged metal 101 a. Theability to avoid the propagation of the cracking to the base materialeven if the cracking occurs in the discharged metal 101 a according tothe method according to the present embodiment is particularly usefulwhen the tensile strength of the metal strip 1 is 700 MPa or more.

According to such a method for manufacturing the electric resistancewelded metal pipe and the electric resistance welded metal pipe, theside ends are butted and welded to each other such that the inclinedsurface 10 b remains on the excess metal 102 a of the metal pipe 2 afterelectric resistance welding and the discharged metal 101 a is not weldedto the excess metal 102 a, so that the propagation of the cracking tothe base material can be avoided even if the cracking is generated inthe discharged metal 101 a. This configuration is particularly usefulwhen the tensile strength of the metal strip 1 is 700 MPa or more.

In applications requiring high strength (high hardness), the electricresistance welded metal pipe may be subjected to a heat treatment afterbeing processed by a component producer. By using an electric resistancewelded metal pipe with good hardenability, which can be adjusted to apredetermined strength (hardness) by a heat treatment at a relativelylow temperature for a relatively short period of time, the componentproducer can reduce heat treatment costs. It is useful to adjust achemical composition of the metal strip 1 and the base material of themetal pipe body in order to obtain the electric resistance welded metalpipe with good hardenability as well as bendability. Using the metalstrip 1 having a composition in which a chemical composition of the basematerial includes from 0.20 to 0.40% by mass of C, from 0.05 to 0.45% bymass of Si, from 1.0 to 1.5% by mass of Mn, from 0.05 to 0.40% by massof Cr, 0.03% by mass or less of P, 0.025% by mass or less of S, from0.0005 to 0.006% by mass of B and from 0.01 to 0.06% by mass of Ti, thebalance being Fe and unavoidable impurities, the side ends of the metalstrip are butted and welded to each other such that the inclined surface10 b remains on the excess metal 102 a of the metal pipe after electricresistance welding and the discharged metal 101 a is not welded to theexcess metal 102 a, whereby the electric resistance welded metal pipehaving good bendability and good hardenability can be obtained.

<C: From 0.20 to 0.40% by Mass>

C is an element effective for strengthening. When the C content is lessthan 0.20% by mass, sufficient strength cannot be obtained. On the otherhand, when the C content is more than 0.40% by mass, the workability islowered. The range of the C content is preferably from 0.24 to 0.37% bymass.

<Si: From 0.05 to 0.45% by Mass>

Si is an element that contributes to improvement of strength by solidsolution. When the Si content is less than 0.05% by mass, sufficientstrength cannot be obtained. On the other hand, when the Si content ismore than 0.45% by mass, toughness and weldability tend to decrease. Therange of the Si content is preferably from 0.1 to 0.4% by mass.

<Mn: From 1.0 to 1.5% by Mass>

Mn is an element effective for improving strength and hardenability.When the Mn content is less than 1.0% by mass, no remarkable additioneffect can be obtained. On the other hand, when the Mn content is morethan 1.5% by mass, toughness of the weld portion tends to decrease. Therange of the Mn content is preferably from 1.10 to 1.40% by mass.

<Cr: From 0.05 to 0.40% by Mass>

Cr is an element effective for improving tempering softening resistanceof the welded heat affected portion. When the Cr content is less than0.05% by mass, no remarkable addition effect can be obtained. On theother hand, when the Cr content is more than 0.40% by mass, weldabilityis decreased. The range of the Cr content is preferably from 0.10 to0.30% by mass.

<P: From 0.03% by Mass or Less>

P is an element that adversely affects toughness. When the P content ismore than 0.03% by mass, a decrease in toughness will be remarkable. Therange of the P content is preferably 0.025% by mass or less. Inaddition, the content of P does not include zero.

<S: 0.025% by Mass or Less>

S is an element that adversely affects toughness. When the S content ismore than 0.025% by mass, a decrease in toughness will be remarkable.The range of the S content is preferably 0.020% by mass or less. Inaddition, the content of S does not include zero.

<B: From 0.0005 to 0.006% by Mass>

B is an element effective for improving hardenability and toughness by aminer addition amount. When the B content is less than 0.0005% by mass,no remarkable addition effect can be obtained. On the other hand, whenthe B content is more than 0.006% by mass, the effect is almostsaturated. The range of the B content is preferably from 0.001 to 0.005%by mass.

<Ti: From 0.01 to 0.06% by Mass>

Ti has an effect of increasing the effective amount of B that improveshardenability, and requires addition in an amount of at least 0.01% bymass or more. When the Ti content is more than 0.06% by mass,weldability tends to decrease. The range of the Ti content is preferablyfrom 0.02 to 0.05% by mass.

The present inventors manufactured the electric resistance welded metalpipe 2 using metal strips 1 of the following two types (base materials Aand B) according to the method for manufacturing the electric resistancemetal pipe of the present embodiment. The thickness of each basematerial of each metal trip 1 was 4.5 mm, and the outer diameter of eachelectric resistance welded metal pipe 2 was φ30.0 mm. The side ends ofeach metal strip 1 were butted against each other such that the inclinedsurface 10 b remained on the excess metal 102 a of each metal pipe afterelectric resistance welding and the discharged metal 101 a was notwelded to the excess metal 102 a. As a result of subjecting theresulting electric resistance welded pipe 2 to 90° bending, any positionwhere the cracking was propagated to the base material was not observed.

<Base Material A>

C: 0.26% by mass; Si: 0.25% by mass; Mn: 1.25% by mass; Cr: 0.20% bymass; P: 0.01% by mass or less; S: 0.005% by mass or less; Ti 0.02% bymass; B: 0.003% by mass, the balance being Fe and unavoidableimpurities.

<Base Material B>

C: 0.34% by mass; Si: 0.18% by mass; Mn: 1.35% by mass; Cr: 0.14% bymass; P: 0.012% by mass; S: 0.008% by mass; Ti: 0.035% by mass; B:0.004% by mass, the balance being Fe and unavoidable impurities.

DESCRIPTION OF REFERENCE NUMERALS

-   1 metal strip-   2 electric resistance welded metal pipe-   30 crushing roll-   50 heating coil-   10 side end-   10 a inner side corner portion-   10 b inclined surface-   101 a discharged metal-   102 a excess metal

1. A method for manufacturing an electric resistance welded metal pipeby butting side ends of a metal strip against each other and thenwelding the side ends by high frequency heating to manufacture anelectric resistance welded metal pipe, each side end being provided withan inner surface side corner portion located on an inner surface side ofthe electric resistance welded metal pipe, wherein the method comprisesa step of forming an inclined surface at the inner surface side cornerportion before butting the side ends of the metal strip; and wherein theside ends are butted and welded to each other such that the inclinedsurface remains on an excess metal of the metal pipe after electricresistance welding and a discharged metal is not welded to the excessmetal.
 2. The method according to claim 1, wherein the forming of theinclined surface is carried out by pressing a crushing roll against theinner surface side corner portion.
 3. The method according to claim 1,wherein a base material of the metal strip has a tensile strength of 700MPa or more.
 4. The method according to claim 1, wherein the basematerial of the metal strip has a chemical composition comprising from0.20 to 0.40% by mass of C, from 0.05 to 0.45% by mass of Si, from 1.0to 1.5% by mass of Mn, from 0.05 to 0.40% by mass of Cr, 0.03% by massor less of P, 0.025% by mass or less of S, from 0.0005 to 0.006% by massof B and from 0.01 to 0.06% by mass of Ti, the balance being Fe andunavoidable impurities.
 5. An electric resistance welded metal pipe,comprising: a metal pipe body having a butted welded portion; an excessmetal; and a discharged metal, the excess metal and the discharged metalbeing formed on the butted welded portion, wherein the excess metalcomprises an inclined surface extending between a corner of the excessmetal which projects toward an interior of the metal pipe body and aroot of the discharged metal in a cross-section perpendicular to alongitudinal direction of the metal pipe body, and the discharged metalis not welded to the excess metal.
 6. The electric resistance weldedmetal pipe according to claim 5, wherein a base material of the metalpipe body has a tensile strength of 700 MPa or more.
 7. The electricresistance welded metal pipe according to claim 5, wherein a basematerial of the metal pipe body has a chemical composition comprisingfrom 0.20 to 0.40% by mass of C, from 0.05 to 0.45% by mass of Si, from1.0 to 1.5% by mass of Mn, from 0.05 to 0.40% by mass of Cr, 0.03% bymass or less of P, 0.025% by mass or less of S, from 0.0005 to 0.006% bymass of B and from 0.01 to 0.06% by mass of Ti, the balance being Fe andunavoidable impurities.